Inter-domain call routing

ABSTRACT

The present invention provides for routing calls between disparate domains, such as a circuit-switched subsystem and a multimedia subsystem. When a user element is homed in a first domain and roaming in a second domain, an incoming call will arrive at a gateway node in the first domain. As a result, a message identifying the user element and indicating that an incoming call has been received at first gateway node for the first domain is sent to a continuity control function (CCF) residing in the multimedia subsystem. The CCF will create and effect delivery of an inter-domain routing number to the first gateway node. The inter-domain routing number is used by the first gateway node to route the call to the second domain. The inter-domain routing number may be associated with a second gateway node of the second domain.

This application claims the benefit of U.S. provisional patentapplication 60/689,883 filed on Jun. 13, 2005, the disclosure of whichis hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to communications and in particular toestablishing calls over circuit-switched subsystems and packetsubsystems.

BACKGROUND OF THE INVENTION

Packet communications have evolved to a point where voice sessions, orcalls, can be supported with essentially the same quality of service asprovided by circuit-switched communications. Packet communications aregenerally supported over packet subsystems, which were initiallysupported by local area networks, but are now supported by wirelesslocal area networks (WLANs). Using WLAN access, user elements cansupport voice sessions using packet communications while movingthroughout the WLAN. As such, WLAN access provides users the samefreedom of movement within a WLAN as cellular access provides userswithin a cellular environment.

In many instances, the coverage areas provided by WLANs and cellularnetworks are complementary. For example, a WLAN may be establishedwithin a building complex in which cellular coverage is limited. Giventhe localized nature of WLAN coverage, cellular networks could bridgethe coverage gaps between WLANs. Unfortunately, WLAN access technologyis independent of cellular access technology. Cellular networksgenerally support circuit-switched communications, and WLANs supportpacket communications. As such, user elements have been developed tosupport both cellular and WLAN communications using differentcommunication interfaces. With these user elements, users can establishcalls via the cellular network and WLAN using the respectivecommunication interfaces; however, establishing and controlling calls ina first domain is difficult when a user element is homed to a seconddomain. Further, once such calls are established, there is at bestlimited ability to maintain control over the calls and to provideservices associated with the calls.

Accordingly, there is a need for a technique to effectively andefficiently establish calls for a user element over both cellularnetworks and WLANs as well as provide seamless control for establishedcalls between the respective domains.

SUMMARY OF THE INVENTION

The present invention provides for routing calls between disparatedomains, such as a circuit-switched subsystem and a multimediasubsystem. When a user element is homed in a first domain and roaming ina second domain, an incoming call will arrive at a gateway node in thefirst domain. As a result, a message identifying the user element andindicating that an incoming call has been received at first gateway nodefor the first domain is sent to a continuity control function (CCF)residing in the multimedia subsystem. The CCF will create and effectdelivery of an inter-domain routing number to the first gateway node.The inter-domain routing number is used by the first gateway node toroute the call to the second domain. The inter-domain routing number maybe associated with a second gateway node of the second domain.

Once the call is routed to the second gateway node, the CCF may beaccessed to further control routing of the call within the second domainto the user element. If the first domain is a circuit-switched subsystemand the second domain is a multimedia subsystem, the CCF will obtain anaddress associated with the user element in the multimedia subsystem andsend a session request toward the user element to establish the callwith the user element. The inter-domain routing number may be createdbased on a user element identifier, an address for the CCF, and a callor session reference identifier.

If the first domain is a multimedia subsystem and the second domain is acircuit-switched subsystem, the CCF will obtain a user elementidentifier based on the inter-domain routing number and sendinstructions for the second gateway to route the call to the userelement via the circuit-switched subsystem. The instructions may includethe user element identifier, such as a Mobile Subscriber IntegratedServices Digital Network number (MSISDN). To initially determine how toroute the call, the CCF may access one or more location registersassociated with the circuit-switched subsystem and the multimediasubsystem to determine that the user element is roaming in thecircuit-switched subsystem, even though the user element is homed to themultimedia subsystem.

Those skilled in the art will appreciate the scope of the presentinvention and realize additional aspects thereof after reading thefollowing detailed description of the preferred embodiments inassociation with the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The accompanying drawing figures incorporated in and forming a part ofthis specification illustrate several aspects of the invention, andtogether with the description serve to explain the principles of theinvention.

FIG. 1 is a communication environment illustrating a circuit-switchedsubsystem and a multimedia subsystem, which provide access for a userelement, according to one embodiment of the present invention.

FIG. 2 shows a communication flow illustrating terminating an incomingcall via the circuit-switched subsystem when the user element is homedin the circuit switched system and roaming in multimedia subsystemaccording to one embodiment of the present invention.

FIGS. 3A and 3B show a communication flow illustrating terminating anincoming call via the multimedia subsystem when the user element ishomed in the multimedia system and roaming in circuit-switched subsystemaccording to one embodiment of the present invention.

FIG. 4 shows a communication flow illustrating originating a call viathe circuit-switched subsystem according to one embodiment of thepresent invention.

FIG. 5 is a block representation of a service node according to oneembodiment of the present invention.

FIG. 6 is a block representation of a user element according to oneembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments set forth below represent the necessary information toenable those skilled in the art to practice the invention and illustratethe best mode of practicing the invention. Upon reading the followingdescription in light of the accompanying drawing figures, those skilledin the art will understand the concepts of the invention and willrecognize applications of these concepts not particularly addressedherein. It should be understood that these concepts and applicationsfall within the scope of the disclosure and the accompanying claims.

The present invention provides for routing calls for a user elementbetween a cellular network and a multimedia subsystem (MS), such as theInternet Protocol (IP) Multimedia Subsystem (IMS). For clarity andconciseness, a cellular network providing circuit-switchedcommunications is referred to as circuit-switched subsystem (CS), and aWLAN providing packet communications is assumed to be part of orassociated with the MS. A public switched telephone network (PSTN) maybe operatively connected to the CS and MS.

The MS and CS are generically referred to as domains, and the presentinvention operates to route calls from one domain to another dependingon the domain where the user element is homed and the domain where aroaming user element is currently registered. A continuity controlfunction (CCF) in the MS may be employed to facilitate such routing ineither direction. For example, when a roaming user element is homed inthe CS and registered in the MS, the CCF may be used to provide the CSwith a routing number for an incoming call. The routing number isconfigured to allow the CS to route the call to the MS, which will thenroute the call to the user element. Alternatively, when a roaming userelement is homed in the MS and registered in the CS, the CCF may be usedto provide the MS with a routing number for an incoming call. Therouting number is configured to allow the MS to route the call to theCS, which will then route the call to the user element. Various entitiesmay be employed to assist in call routing.

When the user element is homed in the MS, call control for originatingand terminating calls in the CS or MS as well as transferring callsbetween the CS and MS may be anchored at the CCF in the MS. Callsignaling for the call is passed through the CCF. The CCF is a serviceprovided in the user element's MS and anchors the user element's activeCS calls and MS sessions to enable mobility across the CS and MS whilemaintaining CS calls or MS sessions. The CCF is addressable using publicservice identities (PSI). In the CS, a directory number associated withthe CCF is used for routing call signaling messages within the CS. Inthe MS, a uniform resource location (URL) associated with the CCF isused for routing call signaling messages within the MS. When the userelement is homed in the CS, normal call signaling may be anchored at themobile switching center to which the user element is homed.

In general, wireless communication techniques having relatively limitedrange, such as WLAN techniques, are referred to as local wirelesscommunication techniques. Thus, local wireless communication techniquessupport packet-based communications, wherein cellular communicationtechniques will generally support circuit-switched communications.Further, the wireless access for local wireless techniques are of alimited range with respect to cellular access techniques. Prior todelving into the details of the present invention, an overview of acommunication environment in which the present invention may be employedis provided.

Turning now to FIG. 1, a communication environment 10 is illustratedaccording to one embodiment of the present invention. In thecommunication environment 10, an MS 12 and a visited CS 14 supportcommunications for a user element 16. The user element 16 includes a CSclient 18 and an MS client 20, which are communication clientsconfigured to support circuit-switched communications via the CS 14 aswell as packet communications via the MS 12, respectively. Forcommunications within the CS 14, either a gateway mobile switchingcenter (GMSC) 22G or a visited mobile switching center (VMSC) 22V willsupport circuit-switched communications for the user element 16.

When the user element 16 is roaming in the MS 12 and is homed in the CS14, incoming calls for the user element 16 are initially routed to theGMSC 22G, which will route the call towards the MS 12. When the userelement 16 is roaming into the CS 14 and homed in the MS 12, incomingcalls for user element 16 are ultimately routed to the VMSC 22V, whichwill connect the call with the user element 16. Both the VMSC 22V andthe GMSC 22G may interact with the MS 12 via a media gateway controller(MGC) 24 and an associated media gateway (MG) 26, both of which areaffiliated with the MS 12.

The MS 12 may include various functions or entities, including aninterrogating call/session control function (I-CSCF) 28I, a servingcall/session control function (S-CSCF) 28S, a CCF 30, a home locationresource (HLR) 32, and a home subscriber service (HSS) 34. Notably, theHLR 32 and the HSS 34 may be implemented in the same or separate nodesand may be accessible by via the MS 12 and the CS 14. The I-CSCF 28I andthe S-CSCF 28S in the MS 12 generally act as Session Initiation Protocol(SIP) proxies and provide various functions in association with callcontrol, as will be appreciated by those skilled in the art. Inoperation, an I-CSCF 28I may interact with the HSS 34 to identify theS-CSCF 28S that is assigned to support a given user element 16. For thepresent invention, the HSS 34 may maintain an association between a userelement 16 and a particular CCF 30 that is assigned to the user element16. As such, the HSS 34 will assist in identifying an S-CSCF 28S for theuser element 16, as well as keep an association between a particular CCF30 and the user element 16. The CCF PSI for the user element 16 may beprovisioned in the user element 16 to enable the user element 16 toinitiate transfers and the like controlled by the CCF 30. Alternatively,the CCF PSI may be transferred to the user element 16 upon networkregistration. Further, the CCF 30 may be a service provided by anapplication server associated with the S-CSCF 28S.

Depending on whether the user element 16 is registered in the MS 12,different techniques may be used to access the MS 12. When the userelement 16 is registered in the MS 12, the user element 16 will have anS-CSCF 28S assigned to it, and will use that S-CSCF 28S to access theCCF 30. When the user element 16 is not registered in the MS network 12,a temporary S-CSCF 28S may be assigned to the user element 16, and thetemporary S-CSCF 28S will be used to access the CCF 30. Regardless ofwhere the user element 16 is homed or is roaming, incoming calls may beoriginated from and outgoing calls may be terminated in the PSTN 36,which is operatively connected to the MS 12 and the CS 14.

In one embodiment of the present invention, the user element 16 isassociated with a published directory number (or address), such as a anMSISDN, which is used by other parties to initiate calls to the userelement 16. A CCF PSI is a published directory number associated withthe CCF 30 and used to route incoming or outgoing calls associated withthe user element 16 to the CCF 30 for call processing. An IP MS routingnumber (IMRN) is a routing number used to route calls into the MS 12from the CS 14. The IMRN may have multiple components. For example, theIMRN may include a user element ID, the CCF PSI, and a call/sessionreference number. A CS routing number (CSRN) is a routing number used toroute calls into CS 14 from the MS 12. The CCF 30 may be configured tomanage the IMRN, the CSRN, or both.

When the user element 16 is homed in the CS 14 and roaming in the MS 12,an incoming call received from the PSTN 36 is initially presented to theGMSC 22G. In an effort to obtain routing information, the GMSC 22G willaccess the HLR 34. The HLR 34 is configured to direct the GMSC 22G tothe CCF 30, which will provide the GMSC 22G with the IMRN. The GMSC 22Guses the IMRN to route the call to the media gateway controller 24,which can use the IMRN to route the call to the user element 16 via theCCF 30. In an alternative embodiment, the HLR 34 may be configured toprovide the IMRN to the GMSC 22G.

With reference to FIG. 2, a communication flow is provided wherein theuser element 16 is homed in the CS 14 and roaming in the MS 12.Initially, a call originating in the PSTN 36 arrives at the GMSC 22G.Arrival of the incoming call is signified when the GMSC 22G receives anInitial Address Message (IAM) having the MSISDN of the user element 16(step 100). The GMSC 22G will query the HLR 32 using the MSISDN intraditional fashion to obtain routing instructions for the call (step102). In this instance, the HLR 32 will recognize that the user element16 is currently being served by the MS 12, and as such will returninstructions for the GMSC 22G to access the CCF 30 for call processinginformation (step 104). The GMSC 22G will send a CCF query with theMSISDN to the CCF 30 (step 106), which will provide the IMRN (step 108)and send the IMRN back to the GMSC 22G in a Connect message (step 110).As noted above, the IMRN may have various components, including anidentifier for the user element 16, the CCF PSI, and the session/callreference information. The Connect message with the IMRN triggers theGMSC 22G to send an IAM to the media gateway controller 24 that isassociated with the IMRN (step 112). The media gateway controller 24 maybe associated with various ranges of IMRNs.

In response, the media gateway controller 24 will generate a SIP Inviteor other session initiation message having the IMRN, and send the Inviteto an appropriate I-CSCF 28I (step 114). To identify the applicationservice providing the CCF 30, the I-CSCF 28I will access the HSS 34using the IMRN. Accordingly, the I-CSCF 28I will send a location queryhaving the IMRN to the HSS 34 (step 116), which will recognize that theIMRN is associated with the CCF 30. As such, the HSS 34 will sendinstructions for the I-CSCF 28I to access the CCF 30 (step 118). TheI-CSCF 28I will then send an Invite having the IMRN to the CCF 30 usingthe appropriate CCF PSI (step 120).

At this point, the CCF 30 will insert a back-to-back user agent (B2BUA),release the IMRN, and establish the call with the user element 16 usingthe appropriate uniform resource identifier (URI) (step 122). The URI isassociated with the MS client 20 of the user element 16 while it isbeing served by the MS 12. The IMRN is a temporary routing number usedto route the call from the GMSC 22G to the CCF 30. Once the call isrouted to the CCF 30, the IMRN is no longer required for the call andcan be reused for other incoming calls to the same or different userelements.

To establish the call with the MS client 20 of the user element 16, theCCF 30 will send an Invite having the URI to the S-CSCF 28S that iscurrently serving the user element 16 (step 124). The S-CSCF 28S willthen forward the Invite having the URI to the user element 16 (step126). At this point, the requisite SIP messaging is passed back andforth between the media gateway controller 24 and the MS client 20 ofthe user element 16 via the CCF 30, S-CSCF 28S, and the I-CSCF 28I toestablish a packet bearer path between the media gateway 26 and the MSclient 20 of the user element 16. In the meantime, the GMSC 22G willestablish a circuit-switched bearer path between the calling party'sdevice and the media gateway 26 via the GMSC 22G. The CS and MS bearerpaths are connected by the media gateway 26 to form the overall bearerpath between the called party's device and the MS client 20 of the userelement 16 (step 128).

The B2BUA is provided by the CCF 30 to terminate a signaling leg towardthe calling party's device and establish another signaling leg towardthe MS client 20 of the user element 16. Subsequently, the CCF 30 maycoordinate call signaling between the two signaling legs. Although aB2BUA is a SIP agent, those skilled on the art will recognize otheragents with similar functionality in other call or session controlprotocols. Notably, the terms “call” and “session” are usedinterchangeably to cover any type of media session.

When the user element 16 is homed in the MS 14 and roaming in the CS 12,an incoming call received from the PSTN 36 is initially presented to themedia gateway controller 24. The media gateway controller 24 may appearas a GMSC on behalf of the MS 12 to which the user element 16 is homed.The media gateway controller 24 will route the call toward the CCF 30via the I-CSCF 28I and the S-CSCF 28S. The CCF 30 will determine thatthe call should be routed into the CS 14 and provide a CSRN to the mediagateway controller 24 for routing the call to the GMSC 22G. Using theCSRN, the media gateway controller 24 will route the call to the GMSC22G. The GMSC 22G may then access the HLR 34 to obtain routinginformation leading to the VMSC 22V currently serving the user element16.

An exemplary communication flow for such a scenario is illustrated inFIGS. 3A and 3B. Initially, an incoming call from the PSTN 36 isreceived at the media gateway controller 24, which acts as a GMSC forthe MS 12. The call arrives in the form of an IAM having the MSISDNassociated with the user element 16 (step 200). In response, the mediagateway controller 24 will initiate an Invite having the MSISDN to theI-CSCF 28I (step 202). The I-CSCF 28I will provide a location query tothe HSS 34 to obtain the identity of the S-CSCF 28S currently servingthe user element 16 (step 204). Since the user element 16 is notcurrently in the MS 12, the HSS 34 will respond with the attributesrequired for selection of an I-CSCF by the I-CSCF 28I (step 204). TheHSS 34 may also provide the URI associated with the MS client 20 of theuser element 16. The I-CSCF 28I will identify an S-CSCF 28S based on theattributes and send an Invite having the URI of the user element 16 tothe selected S-CSCF 28S (step 206). The S-CSCF 28S will access the HSS34 to obtain the subscriber profile associated with the user element 16based on the MSISDN or the URI (step 208). The HSS 34 will recognizethat the user element 16 is not being served by the MS 12 and that theincoming call should be routed to the CCF 30. This information is alsoprovided to the S-CSCF 28S in response to the subscriber profile query(step 208). This access will effectively set the S-CSCF 28S and the HSS34 in association with the user element 16. The S-CSCF 28S will thenforward the Invite having the URI to the CCF 30 (step 210) according tothe subscriber profile.

To identify the location of the user element 16, the CCF 30 willinitially access the HSS 34 to determine the location of the userelement 16 within the MS 12 (step 212). The HSS 34 will indicate thatthe user element 16 is not within the MS 12 (step 212). As such, the CCF30 will access the HLR 32 to determine where the user element 16 iswithin the CS 14 (step 214). In this instance, the CS client 18 of theuser element 16 is being served by the VMSC 22V, which has registeredwith the HLR 32 as the currently serving MSC for the user element 16.Accordingly, the HLR 32 will indicate that the user element 16 isroaming in the CS 14 (step 214). In response, the CCF 30 will determineto route the call via the CS 14 and assign a CSRN, which corresponds tothe GMSC 22G of the CS 14 (step 216). The CCF 30 may send a SIP 302Moved Temporarily message having the CSRN to the S-CSCF 28S (step 218),which will forward the 302 Moved Temporarily message to the I-CSCF 28I(step 220). The I-CSCF 28I will further forward the 302 MovedTemporarily message to the media gateway controller 24 (step 222). Themedia gateway controller 24 will then initiate an IAM having the CSRNtoward the GMSC 22G (step 224). An advanced intelligent network (AI)trigger may be used to identify an incoming IAM having the CSRN (step226). In response to the AI trigger, the GMSC 22G will be configured tosend a CCF query having the CSRN to the CCF 30 to obtain the MSISDN forthe user element 16 (step 228). The CCF 30 will map the CSRN to theMSISDN, which was provided in an earlier Invite (step 230), and providethe MSISDN for the user element 16 to the GMSC 22G in a Connect message(step 232).

Armed with the MSISDN, the GMSC 22G can now send an HLR query to the HLR32 to obtain routing information for the user element 16 (step 234). TheHLR 32 will provide a CS identifier (CSI), which is configured toinstruct the GMSC 22G to once again access the CCF 30 (step 236). TheGMSC 22G will then update the CCF 30 (step 238), which will update thesession state for the incoming call (step 240). The CCF 30 will thensend a Continue message back to the GMSC 22G (step 242), which willprovide another HLR query having the CSI (step 244). To obtain routinginformation for the call, the HLR 32 will send a Mobile SubscriberRouting Number (MSRN) query to the VMSC 22V to obtain a temporary MSRNfor the user element 16 (step 246). The temporary MSRN is a routingnumber that is temporarily assigned to the user element for routing ofthe session to the VMSC 22V. The VMSC 22V will send the MSRN back to theHLR 32 in an Acknowledgement message (ACK) (step 248). The HLR 32 willforward the MSRN to the GMSC 22G in another Acknowledgement message(step 250).

At this point, the GMSC 22G has the temporary MSRN for the user element16, and will send an IAM to the VMSC 22V (step 252). The VMSC 22V willinitiate alerting of the user element 16 (not shown). After anyrequisite call signaling, a bearer path is established between the CSclient 18 of the user element 16 and the called party's device via theGMSC 22G and the VMSC 22V (step 254). The call signaling path may bemaintained through the CCF 30, S-CSCF 28S, I-CSCF 28I, and the mediagateway controller 24 if so desired, in addition to the GMSC 22G and theVMSC 22V.

With reference to FIG. 4, a communication flow is provided to illustrateorigination of a call toward the PSTN 36 from the CS client 18 of theuser element 16 via the CS 14. Initially, the CS client 18 will send acall setup message to its supporting VMSC 22V (step 300). In response,the VMSC 22V will obtain the IMRN from the CCF 30 or generate the IMRNbased on provisioned information (step 302). The IMRN may identify theremote endpoint, the CCF PSI, and the call/session referenceinformation. With the IMRN, the VMSC 22V will send an IAM having theIMRN to the media gateway controller 24 (step 304) and a Call Proceedingmessage back to the CS client 18 of the user element 16 (step 306). Thisexchange results in a CS bearer leg being established from the CS client18 to the media gateway 26 via the VMSC 22V.

The media gateway controller 24 acts as a gateway to the MS 12 for theCS 14, and as a user agent on behalf of the user element 16 in the MS12. Upon receiving the IAM from the VMSC 22V, the media gatewaycontroller 24 will send an Invite having the IMSC to the S-CSCF 28S viathe I-CSCF 28I to initiate establishment of a bearer leg through the MS12 and between a remote endpoint and the media gateway 26 (step 308).The Invite may also indicate that the call was originated through the CS14. The S-CSCF 28S will recognize the need to invoke the CCF 30 for thecall, and will send the Invite to the CCF 30 (step 310), which willinvoke a back-to-back user agent (B2BUA) and then take the necessarysteps to complete the call (step 312). The CCF 30 will obtain ordetermine the ID of the user element 16 as well as the remote endpointor destination address based on the IMRN.

The CCF 30 will send an Invite back to the S-CSCF 28S to complete thecall (step 314). The Invite will now include the address of the remoteendpoint or a supporting node with which a packet session can beestablished. The Invite will identify the media gateway controller 24 ofthe media gateway 26 as the other endpoint for the packet session thatwill support the call. The S-CSCF 28S will then send the Invite towardthe remote endpoint 36 (step 316). At this point, the traditionalsession message exchange between the remote endpoint and the mediagateway controller 24 will take place through the CCF 30 and the S-CSCF28S to prepare the respective remote endpoint and media gateway 26 tosupport the bearer path through the MS 12. The bearer path through theCS 14 is established between the media gateway 26 and CS client of theuser element 16. As a result, a bearer path is established between theremote endpoint and the user element 16 via the VMSC 22V and the mediagateway 26 (step 318).

With reference to FIG. 5, a service node 44 is provided according to oneembodiment of the present invention. The service node 44 may reside inthe MS 12 and include a control system 46 and associated memory 48 toprovide the functionality for any one or a combination of the following:the CCF 30, the I-CSCF 28I, and the S-CSCF 28S. The control system 46will also be associated with a communication interface 50 to facilitatecommunications with any entity affiliated with the MS 12 orappropriately associated networks.

With reference to FIG. 6, a block representation of a user element 16 isprovided. The user element 16 may include a control system 52 havingsufficient memory 54 to support operation of the CS client 18 and the MSclient 20. The control system 52 will cooperate closely with acommunication interface 56 to allow the CS client 18 and the MS client20 to facilitate communications over the CS 14 or the MS 12 as describedabove. The control system 52 may also be associated with a userinterface 58, which will facilitate interaction with a user. The userinterface 58 may include a microphone and speaker to facilitate voicecommunications with the user, as well as a keypad and display to allowthe user to input and view information.

Those skilled in the art will recognize improvements and modificationsto the preferred embodiments of the present invention. All suchimprovements and modifications are considered within the scope of theconcepts disclosed herein and the claims that follow.

What is claimed is:
 1. A method for routing an incoming call, the methodcomprising: receiving in a first domain, by a continuity controlfunction (CCF) executing on an application server from a serving callsession control function (S-CSCF), an invite message identifying a userelement; terminating the invite message on a first signaling leg in thefirst domain by the CCF functioning as a Back-to-Back User Agent (B2BUA)on the application server; sending, by the CCF to a home locationregister (HLR), a first message identifying the user element andindicating that an incoming call intended for the user element has beenrouted to the CCF in the first domain; receiving, by the CCF, aninter-domain routing number for the CCF to use to route the incomingcall into a second domain, wherein the inter-domain routing numberidentifies a visited mobile switching center (VMSC) in the seconddomain; and sending, by the CCF functioning as the B2BUA on theapplication server, a second message on a second signaling leg, wherethe second message is used to route the incoming call to the seconddomain using the interdomain routing number that identifies the VMSC,obtained in the step of receiving the interdomain routing number,wherein the second domain is a circuit-switched subsystem and the firstdomain is a multimedia subsystem.
 2. The method of claim 1, wherein theCCF provides the inter-domain routing number to route the incoming callinto the second domain from the first domain, and wherein theinter-domain routing number is released after routing the incoming call.3. The method of claim 1, comprising receiving a third messageindicating that the incoming call has been routed to a gateway node forthe second domain and effecting routing of the incoming call to the userelement via the gateway node.
 4. The method of claim 3, whereineffecting routing of the incoming call to the user element furthercomprises obtaining an address associated with the user element in thecircuit-switched subsystem and sending a session request toward the userelement.
 5. The method of claim 1, further comprising identifying alocation of the user element and determining to route the incoming callto the user element via the second domain based on the location.
 6. Themethod of claim 1, further comprising anchoring call signaling for theincoming call in the multimedia subsystem.
 7. The method of claim 2,wherein the invite message is received and the inter-domain routingnumber is provided from the circuit-switched subsystem.
 8. The method ofclaim 1, wherein the application server comprises a media gatewaycontroller associated with a media gateway to the multimedia subsystemfrom the circuit-switched subsystem.
 9. An application server,comprising: a processor configured to execute a continuity controlfunction (CCF) configured to: receive an invite message in a firstdomain from a serving call session control function (S-CSCF), the invitemessage identifying a user element; terminate the invite message on afirst signaling leg in the first domain, wherein the CCF is configuredto function as a Back-to-Back User Agent (B2BUA) to terminate the invitemessage on the first signaling leg; send, by the CCF to a home locationregister (HLR), a first message identifying the user element andindicating that an incoming call intended for the user element has beenrouted to the CCF in the first domain; receive, by the CCF, aninter-domain routing number for the CCF to use to route the incomingcall into a second domain, wherein the inter-domain routing numberidentifies a visited mobile switching center (VMSC) in the seconddomain; and send, by the CCF functioning as the B2BUA on the applicationserver a second message on a second signaling leg, where the secondmessage is at least partially used to route the incoming call to thesecond domain using the interdomain routing number that identifies theVMSC, obtained in the step of receiving the interdomain routing number,wherein the second domain is a circuit-switched subsystem and the firstdomain is a multimedia subsystem.
 10. The application server of claim 9,wherein the CCF is further configured to: provide the inter-domainrouting number to route the incoming call into the second domain fromthe first domain; and release the inter-domain routing number afterrouting the incoming call.
 11. The application server of claim 10,wherein the CCF is further configured to: receive a third messageindicating that the incoming call has been routed to a gateway node forthe second domain; and effect routing of the incoming call to the userelement via the gateway node.
 12. The application server of claim 11,wherein to effect routing of the incoming call to the user element viathe gateway node, the CCF is further configured to: obtain an addressassociated with the user element in the circuit-switched subsystem; andsend a session request toward the user element.
 13. The applicationserver of claim 9, wherein the CCF is further configured to identify alocation of the user element and determine to route the incoming call tothe user element via the second domain based on the location.
 14. Theapplication server of claim 9, wherein the CCF is further adapted toanchor call signaling for the incoming call in the multimedia subsystem.15. The application server of claim 10, wherein the invite message isreceived in and the inter-domain routing number is provided from thecircuit-switched subsystem.
 16. The application server of claim 9,wherein the application server comprises a media gateway controllerassociated with a media gateway to the multimedia subsystem from thecircuit-switched subsystem.
 17. The method of claim 1, wherein themultimedia system is an internet protocol multimedia system.
 18. Theapplication server of claim 9, wherein the multimedia system is aninternet protocol multimedia system.
 19. The application server of claim9, wherein the B2BUA terminates the first signaling leg of the incomingcall toward a calling party's device and establishes the secondsignaling leg toward a client of the user element in the second domain.20. The application server of claim 14, wherein anchoring call signalingfor the incoming call in the multimedia subsystem comprises terminatingan incoming session from a calling device to the multimedia subsystemand initiating an outgoing session to the circuit-switched subsystembased on the terminated incoming session and the inter-domain routingnumber.